Today we will build a circuit that detects the interruption of an infrared beam allowing an alarm to be created.

The LM358IC has two comparators. Pin 3 which is the non-inverted end of the voltage comparator is connected to the photodiode and the inverted pin (pin 2) is connected to a 10K variable resistor. The output of the comparator is connected to a resistor and in turn to an LED.

When the infrared beam is reflected in the obstacle and picked up by the photodiode, the voltage at the non-inverted end of the voltage comparator is greater than at the inverted end and the output of the comparator will be high. As the comparator output is connected to the LED, when the LED is high it means there are no obstacles.

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